1. Field of the Invention
The present invention relates to a method of purifying the exhaust gas of internal combustion engines and, more specifically, to a method of purifying the exhaust gas by using a NOx occluding and reducing catalyst.
2. Description of the Related Art
There is known a method of purifying the exhaust gas of an internal combustion engine by disposing a NOx occluding and reducing catalyst in an exhaust gas passage of the internal combustion engine to selectively occlude and hold NOx in the exhaust gas by adsorption, by absorption or by both of them when the air-fuel ratio of the exhaust gas flowing in is lean, and to reduce the occluded NOx with reducing components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in becomes the stoichiometric air-fuel ratio or a rich air-fuel ratio, and by causing the NOx occluding and reducing catalyst to occlude the NOx in the exhaust gas from the engine when the engine is operated at a lean air-fuel ratio, and purifying the NOx occluded by the NOx occluding and reducing catalyst by reduction with reducing components in the exhaust gas from the engine when the engine is operated at the stoichiometric air-fuel ratio or at the rich air-fuel ratio.
In the method of purifying exhaust gas by using the NOx occluding and reducing catalyst, a problem often arises in regard to a drop in the NOx purifying capability of the NOx occluding and reducing catalyst due to the occlusion of sulfur in the fuel, i.e., sulfur contamination. Fuel such as gasoline or diesel oil contains sulfur in small amounts and, hence, trace amounts of SOx (sulfur oxides such as SO2, SO3, etc.) are contained in the exhaust gas after the combustion. The SOx in the exhaust gas is occluded by the NOx occluding and reducing catalyst based on the same mechanism as for the NOx. However, once occluded, the SOx is not released from the NOx occluding and reducing catalyst under a temperature condition where the NOx occluded by the NOx occluding and reducing catalyst can be purified by reduction.
When the exhaust gas contains SOx, therefore, the amount of SOx occluded by the NOx occluding and reducing catalyst gradually increases after the repetition of occlusion and reduction of NOx, and the amount of NOx that can be occluded decreases by an amount of the SOx that is occluded. Namely, the SOx causes a decrease in the NOx occluding capability or NOx purifying capability of the NOx occluding and reducing catalyst, i.e., the so-called sulfur contamination (or S contamination) occurs.
Once the sulfur contamination takes place on the NOx occluding and reducing catalyst, the NOx purifying capability must be recovered by carrying out the operation at a rich air-fuel ratio while elevating the catalyst temperature to be considerably higher than that usual for purifying the NOx by reduction. When the operation continues for long periods of time in a low-speed and low-load region where the exhaust gas temperature of the engine is low, therefore, the temperature of the exhaust gas must be raised to a considerable degree to remove the SOx contamination. Thus, there arise such problems that the SOx contamination is not removed to a sufficient degree and the fuel is consumed in increased amounts for removing the SOx contamination.
In order to solve these problems, there has been devised a method of preventing the occurrence of SOx contamination by suppressing the occlusion of SOx in the exhaust gas by the NOx occluding and reducing catalyst.
For example, Japan se Patent No. 3154105 discloses a method of suppressing the occlusion by the NOx occluding and reducing catalyst by mixing an additive to the fuel thereby to convert gas components such as SO2 and SO3 in the exhaust gas into solid sulfates.
According to the method of '105 patent, an additive containing a metal compound such as barium is added to the fuel which is, then, supplied into the combustion chamber of an engine and is burned; i.e., the additive reacts with sulfur in the fuel and forms a sulfate (e.g., barium sulfate BaSO4) in the form of solid particles.
Thus formed sulfate is in the form of solid particles and is not occluded by the NOx occluding and reducing catalyst unlike the gaseous sulfur oxide. In the method of '105 patent, therefore, the amount of SOx occluded by the NOx occluding and reducing catalyst does not increase, and the NOx purifying capability of the NOx occluding and reducing catalyst does not decrease (sulfur contamination does not occur) even after the NOx occluding and reducing catalyst is used for extended periods of time.
However, a method such as that disclosed in the '105 patent which decreases SOx in the exhaust gas by adding, to the fuel, an additive that forms a solid sulfate upon reacting with sulfur during the combustion, sometimes causes problems.
According to the method of '105 patent, for example, an additive is added to the fuel. That is, the additive must be mixed into the fuel beforehand, or the additive must be supplied separately from the fuel and be added into the fuel before it is supplied to the engine. Apart from the case where fuel premixed with the additive is supplied, it is desired that the interval for supplying the additive is lengthened as much as possible by minimizing the consumption of the additive, if the additive is supplied separately from the fuel. According to the method of '105 patent, however, no consideration has been given to suppressing the consumption of the additive.
Besides, the solid sulfate formed by the additive is in the form of fine particles, and is not trapped by the NOx occluding and reducing catalyst but is released into the atmosphere passing through the catalyst. Therefore, there arises a problem of an increase in the total amount of the particulate matter emitted from the engine if the additive is used even when it is not truly needed.
This problem occurs either when there is used the fuel into which the additive has been mixed already or when the fuel and the additive are separately supplied.
It has further been known that when an additive such as a metal compound is mixed into the fuel, there takes place knocking due to a decrease in the octane value of the fuel, as will be described later, or an increased deposit forms on the combustion chambers and on the exhaust valves of the engine.
Therefore, the above-mentioned problems must be solved when an additive such as a metal compound that forms a solid sulfate in the exhaust gas upon the reaction with sulfur is added to the fuel as in '105 patent. However, '105 patent does not give consideration to these problems.